PET imaging device for observing the brain

ABSTRACT

A PET imaging device for observing a brain includes a hollow three-dimensional structure with a shape capable of housing a head. The PET imaging device comprising multiple independent gamma ray detection modules that together form a structure capable of surrounding the head, said detection modules comprise continuous scintillation crystal blocks, wherein “continuous” means that the crystal blocks can be continuous in one or in two directions, each of the continuous scintillation crystal blocks has a polygonal main cross-section, and said structure is an elongated structure having a major axis in a direction corresponding to the front-nape direction and a shorter axis in a direction corresponding to a straight line joining ears on the head. The continuous scintillation crystal blocks are positioned adjacent to fit laterally in an exact manner with each other throughout their entire thickness, building a mosaic, without gaps between adjacent crystal blocks and without overlapping each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of and claims priority toU.S. application Ser. No. 16/169,075, filed on Oct. 24, 2018, which is,in turn, a continuation of and claims priority to PCT Application No.PCT/ES2017/070248, filed on Apr. 25, 2017, which, in turn, claimspriority to Spanish Application No. P201630524, filed on Apr. 25, 2016.The entire contents of each of these applications is incorporated hereinby reference.

FIELD OF THE INVENTION

The present invention has application in the field of medical devicesfor diagnostic imaging, specifically in positron emission tomography(PET) devices.

BACKGROUND OF THE INVENTION

Positron Emission Tomography is an “in vivo” diagnostic and researchtechnique for imaging, capable of measuring the metabolic activity ofthe human body. The PET technique is based on detecting and analyzingthe three-dimensional distribution that an ultrashort half-liferadiopharmaceutical, administered through an intravenous injection,takes inside the body. Depending on what you want to study, differentradiopharmaceuticals are used.

The image is obtained thanks to the fact that the devices are able todetect the gamma photons emitted by the patient. These 511 keV gammaphotons are the product of an annihilation between a positron, emittedby the radiopharmaceutical, and a cortical electron from the patient'sbody. This annihilation gives rise to the emission, essentially, of twophotons. For these photons to end up shaping the image they must bedetected “in coincidence”, that is, at the same time, in an appropriatetime period (nanoseconds). In addition, they must come from the samedirection and opposite sense of direction, but also their energy mustexceed a minimum threshold that certifies that they have not sufferedsignificant energy dispersions in their journey (scatter phenomenon) tothe detectors. The detectors of a PET scanner are arranged in aring-shaped structure around the patient, and because they detect incoincidence the photons generated in each annihilation, they will makeup the image. To obtain the image, these detected photons are convertedinto electrical signals. This information is then subjected to filteringand reconstruction processes, thanks to which the image is obtained.

The dedicated brain PET is useful for the measurement of brain activityand is effective for the early diagnosis of neurodegenerative diseasessuch as Parkinson's disease or Alzheimer's disease, as well as othermental illnesses such as schizophrenia or severe depression. For anaccurate diagnosis high quality images are required, therefore, thedevice must be designed with high spatial resolution and sensitivity.The sensitivity can be improved by increasing the thickness of thecrystals, decreasing the distance of the detector to the patient and/orcovering the maximum possible surface of the patient's skull.

Various approaches have been proposed to solve the problem of improvingthe sensitivity of brain PET, such as patent application WO2010/033159,where Majewski et al. They propose a simple spherical ring around thehead to generate the Image. The invention described in this applicationhas the disadvantages that the ring does not cover the entire brain and,having a circular shape, is not optimized for the typically oval shapeof the human head. Likewise, in 2011, the article by S. Yamamoto et al.is published in the IEEE Transactions on Nuclear Science (vol. 58, pp.668 to 673). “Development of a Brain PET system, PET-Hat: A Wearable PETSystem for Brain Research” where an equally circular and single-ring PETdevice is described, showing no advance in the aspect of sensitivitywith respect to the aforementioned patent. In 2013, Weinberg I. et aldescribed in the patent US2013218010 a multi-ring device of circularsection that includes partial rings of detectors, which do not completethe ring, in order to increase the sensitivity. All these works have incommon that they are based on rings of circular section using squaredetectors.

In 2015, Tashima ef al. describe in the U.S. Pat. No. 9,226,717 B2(US201501 15162 A1) a PET device, of similarly circular section, butorganized in the form of a hemisphere instead of a cylindrical one,which incorporates an element not physically coupled to the main hull inorder to increase its sensitivity.

The construction of a device that optimizes the sensitivity of adedicated brain PET, which at the same time minimizes the number ofdetectors used, would require building a surface that would becompletely adapted, in shape and size, to the head, particularly thehuman head. However, there are important limitations as to how togenerate this surface due to the procedure used to manufacture thecontinuous scintillating crystals that are included in these devices.These limitations are related to the maximum size and shape in whichthese crystals can be carved.

Further, it is impossible to perform exactly a three-dimensionalelongated curved surface (such as an ellipsoid) starting from flatsurfaces in the form of polygons. However, although it is not trivial,it is possible to approximate those curved surfaces to a polyhedronconstructed from flat surfaces in the form of polygons. The object ofthe present invention is precisely to achieve a PET imaging device withthe maximum angular coverage of the brain by means of independentdetection modules of polygonal main section and together constituting athree-dimensional elongated structure adapted to the head, in particularthe human head and with ability to be arranged as close as possible tothe head, to minimize the number of detectors.

Taking into account the limitations existing in the manufacturing ofcrystals, and using the regular geometric shapes previously proposed inthe state of the art, the device is either too far from the actualgeometry of the head (as seen in FIG. 1), or the truncated icosahedronthat illustrates the state of the art), or too small due to the numberof faces and maximum size of each face (typically 70 mm), as in the caseof the truncated icosahedron.

BRIEF DESCRIPTION OF THE INVENTION

The present invention solves the problems of the state of the art bymeans of a PET device dedicated to the brain with a geometric shapedifferent from those previously proposed and which improves thesensitivity of the equipment with respect to other configurations basedon rings or spherical helmets.

Detectors are also proposed with a shape different from the square form(triangular, pentagonal or hexagonal) so that starting from flatsurfaces made by several regular polygons the set ideally approximates asphere.

The device of the present invention comprises detectors with pentagonand hexagon shapes, forming geometries that, unlike other more commonforms, such as the truncated icosahedron, allow generating largerdiameters of the detector device, for a given maximum size ofscintillating crystal. Likewise, they allow generating elongated forms,with non-circular perimeters, better adapted to the morphology of thehead, in particular the human head.

The expression “continuous” crystal in this specification is to beunderstood in a broad sense: “continuous” means that the crystal can becontinuous in one direction or in two directions (similar to a slab),wherein “direction” refers to the direction of one of the axes of aCartesian coordinate system.

“Continuous crystal” means not only crystals with regular cross section,such as squares, but also with shapes in which one dimension issignificantly larger than the other such as crystal slabs orsemi-monolithic crystals.

“Crystal block” in this specification means a single crystal or a set ofseveral crystals. A single continuous crystal or a set of continuouscrystals form a “crystal block”. As an example:

-   -   a “crystal block” can be formed by a single crystal with regular        main cross-section, such as square main cross-section    -   a “crystal block” can be formed by several crystals each one        with a rectangular main cross-section and wherein the resulting        “crystal block” might have a square main cross-section.

According to the invention the “crystal blocks”, that can be formed asexplained above, by one or more crystals, are positioned adjacent one toeach other to fit laterally in an exact manner throughout their entirethickness, building a mosaic, without leaving gaps between adjacent“crystals blocks” and without overlapping each other.

The “crystal blocks” are continuous in one direction, or in twodirections, wherein “direction” refers to the axes of a Cartesiancoordinate system. The crystal or crystals forming a “crystal block” arecontinuous as well.

The essential property defining a “continuous crystal”, in this broadsense, is that the depth of interaction of the gamma rays inside thecrystal is determined by the width or/and other features of the lightdistribution measured with the photo-sensors that are allocated justbelow the crystal and serve as a readout of the scintillated light. Theposition of at least one of the other two coordinates (the ones alongthe photo-sensor matrix) of the total of 3 coordinates of the gamma rayimpact position is also determined by measuring the center of gravityor/and other features of the light distribution with thosephoto-sensors. If only one coordinate is determined in such a way, theother one is determined through the finding of which crystal has beenhit by the gamma ray, as is done in pixelated crystals.

This definition of “continuous crystal” therefore implies that theimpact position of the gamma rays may take any value, both in the depthof interaction and at least one of the other two coordinates, inside thecrystal, varying continuously from, and to, all the spatial limits ofthat crystal. This does not happen with the pixelated crystals since inthis case the detector module is not able to determine differentpositions inside the crystal and the position of all impacts in thatcrystal is usually assigned to the center of that pixel.

The essential feature of the present patent application is thegeometrical configuration used to allocate all the detector modules(also called detector blocks), so that there are no gaps betweenadjacent “crystal blocks”, in order to increase the sensitivity of thebrain scanner. The detector modules may consist of one or a plurality ofcontinuous crystals in the broad sense, as defined previously, and suchplurality of continuous crystals may (or not) share the same read-outelectronics.

Unless otherwise specified in this specification, the informationreferring to a “crystal” or “continuous crystal” applies to “crystalblocks” as well.

Furthermore, in another configuration of the present patent applicationtwo panels formed by detector modules of the brain scanner, may beremoved temporally from the brain scanner so that a different part ofthe body is imaged with those two panels. In a preferred configurationboth panels are of identical shape and are flat. Those panels may beextracted, for instance, from the part of the brain scannercorresponding to the top and back part of the head, or from the partscorresponding to the two lateral sides of the head.

The device of the present invention is a PET imaging device dedicated tothe observation of the brain, characterized in that as a whole it has astructure with a shape capable of housing a head, comprising independentgamma ray detection modules, said detection modules comprise continuousscintillation crystal blocks of polygonal main section, in which thedetection modules together form a hollow three-dimensional structurecapable of enclosing the head, and said elongated three-dimensionalstructure having a major axis in the direction corresponding to thefront-nape direction in the direction corresponding to the straight linejoining the ears, and the adjacent scintillation crystal blocks beingarranged to fit laterally in an exact manner with each other throughouttheir thickness, constituting a mosaic, that is to say, without leavinggaps and without overlapping each other. Preferably the adjacentscintillation crystal blocks are of the same thickness fitting laterallyin an exact manner with one another throughout their entire thickness.

The detection modules may have a square or rectangular shape formingtogether a lying down, hollow, prism with a rectangular base capable ofhousing a head, the anterior base of said prism being at the front ofthe structure capable of being faced to the face of a subject, and therear base of the prism is in the zone that corresponds to the back ofthe head, in the posterior part of the structure that can be confrontedto the occipital bone. Other alternatives for this embodiment aredefined in the dependent claims.

The detection modules may also have a square or rectangular shapeforming together a hollow prism of bases of pentagonal, hexagonal, oroctagonal section capable of housing a head, the anterior base of saidprism being at the front of the structure capable of being facing theface of one subject, and the other base of the prism on the back of thestructure would face the back of the head, that could be confronted withthe occipital bone.

Additional alternatives relate to a PET imaging device comprisingdetection modules that are of triangular, square, rectangular shape orcombination thereof, which together form a hollow prism with a polygonaldome base, for example square, pentagonal, hexagonal, heptagonal oroctagonal.

Additional alternatives refer to a PET imaging device comprising squareor rectangular shaped detection modules, forming together a hollow prismcapable of housing a head whose bases are formed by polyhedral domes.According to another variant, the detection modules have a square orrectangular shape forming together a hollow octahedral prism capable ofhousing a head, and with a base in the form of a square dome.

The detection modules can also form a hollow prism whose lateral facesare the faces capable of being arranged between the nape and theforehead, and the bases of the prism are the faces that can be arrangedparallel to the ears. For example, the prism can be formed by eightsides including a side that can face the chin of a subject and theabsent sides—likely to face the neck and the eye area—and the bases, areformed by a square formed by several detectors, for example 9 detectors.

According to other alternatives, the detector modules can form athree-dimensional hollow structure with an elongated shape, that is alsonarrower in the anterior part corresponding to the area of the forehead,when the device is in use, than in the posterior part corresponding tothe area of the nape, when the device is in use, so that said structureis capable of being adapted to the shape of the head. For example, thedetector modules can form a three-dimensional hollow structurecomprising 76 vertices or 84 vertices, and have flat surfaces ofsection, —which in at least part of said detector modules—has apentagonal or hexagonal section, regular or irregular.

The PET imaging device can further comprise a mechanical matrix withstructure that is opaque to visible light, rigid, honeycomb like, whereeach of the detection modules are housed in their pre-establishedposition and orientation, and a mechanical interface in order that amodule can be linked with the other modules. Therefore, the mechanicalmatrix will cover all the modules, and each one of the modules, in turn,can be encapsulated or not.

In the detection modules, the continuous scintillation crystal blockscan have the same width for all detectors regardless of their polygonalshape.

The PET imaging device can also comprise detector modules with twodifferent sizes.

Alternatively, each detection module has a single face where thephoto-detectors are located, and such that the surface opposite to thephoto-detection is completely polished and covered by a retro-reflector.

The PET imaging device may also comprise a light diffusing sheet or alight guide between each continuous scintillation crystal block and thephoto-detectors.

The detection modules are preferably configured so that the gamma raysenter through the opposite side with respect to the location of thephoto-detectors, although they could also be configured so that thegamma rays enter through the face where the photo-detectors are located.

The PET imaging device may also comprise a protective element capable ofbeing mechanically or manually operated to produce a complete adaptationof the PET imaging device to the shape of an object—in particular, ahuman head—the image of which is to be obtained, and to immobilize saidobject. The protective element can be an air cushion system, attached tothe set of detection modules that by means of an inflation system,filled the separation between the PET image device and the object theimage of which is to be obtained, or a system of elastic containers,filled with small spherical particles or with any other geometry, of lowdensity, which when pressed mechanically cause the adaptation of the PETimaging device to the shape of the object whose image is intended to beobtained.

The invention also has as an object a method for performing an imageacquisition with a PET imaging device as defined above, which comprisesarranging a protective element as the one defined above between the PETimaging device and the object the image of which is to be obtained, (inparticular a human head), so that said protective element when beingmechanically or manually operated produces a complete adaptation of thePET image device to the shape of the object the image of which is to beobtained. Other alternatives for this embodiment are defined in thedependent claims.

In the present specification, unless explicitly stated otherwise, theterm “crystal” is equivalent to the term “continuous crystal”. The sameapplies to “crystal block” and “continuous crystal block”.

The expression “detector module” is equivalent to “detection module” orsimply “module”, unless explicitly stated otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: shows a configuration of detection modules forming multiplerings, of circular section, belonging to the state of the art, comparedwith the morphology of the human head and in which an importantseparation between the detectors and the head can be seen.

FIG. 2: Left: shows a real human head and the line on which the sectionto which the PET device should approach—in order to increase itsefficiency—will be calculated. Right: Shows an ellipsoid representingthe involute of a real human head.

FIG. 3: illustrates an embodiment of the detection module, comprising acontinuous scintillation crystal block, a set of photo-detectors, thereading electronics, and a mechanical encapsulation of the wholeassembly. 1: gamma ray, 2: scintillation light emission point; 3:photo-sensor array; 4: scintillation crystal block; 5: treatment of theglass entrance surface, preferably retroreflector; 6: treatment of thelateral surface of the glass by means of specular reflector, black paintor other absorbent paint, or a combination thereof; 7: light diffuser orlight guide.

FIG. 4: illustrates embodiments of the shape of the continuousscintillation crystal blocks of the detection modules, with polygonalmain section. Prisms of triangular, square, pentagonal, hexagonal,heptagonal and octagonal section. Pyramids truncated by a cut parallelto the base of triangular, square, pentagonal, hexagonal, heptagonal andoctagonal section.

FIG. 5: shows an example of configuration of the PET imaging device inthe form of a lying down, hollow prism with a square base. Said prismalmost completely covers the head leaving only a hole for the neck andfor the eyes. The sides corresponding to the lower area of the jaw onlyhave detectors in that region, in order to leave a loose space for theneck. The rear or rear base has been completely covered with squaredetectors and has even been extended towards the back to increasesensitivity. The front base has been covered only partially, in the areaof the chin and forehead, to allow the eyes a comfortable vision. Thelower side of the prism, which corresponds to the area of the neck whenthe imaging device is in use, only has one fragment so that it forms an“L” with the front base of the prism. The position of the part of theL-shaped front base corresponding to the chin area is adjusted for eachsubject after having placed the PET imaging device on the head.

FIG. 6: shows an exemplary embodiment of detection modules with squarescintillating continuous crystal blocks in the shape of an irregularprism with eight sides. The sides corresponding to the lower area of thejaw only have detectors in that region to leave a loose space for theneck. The rear or posterior base has been completely covered with squaredetectors. The front base has been covered only partially, in the areaof the chin and forehead, to allow the eyes a comfortable vision.

FIG. 7: shows an example of a base in the form of a square dome. Thisbase adapts better to the shape of the back of the head. This baseincludes triangular scintillation crystal blocks. Other similar bases inthe form of a pentagonal, hexagonal dome, etc., are also analogously putinto practice.

FIG. 8: shows an example of arrangement of square scintillatingcontinuous crystal blocks in the form of an irregular eight-sided prism.The back has been completely covered by a square dome. Alternatively,other similar bases are used in the form of a pentagonal, hexagonaldome, etc. Likewise, the front part has also been covered, although onlypartially, in the area of the chin and the forehead, to allow the eyes acomfortable vision, by means of a base of the prism in the form of asquare dome.

FIG. 9: shows a configuration example of the PET imaging device in whichthe bases of the hollow prism are parallel to the ears. In this case theprism is formed by eight sides (octagonal prism), including the sidecorresponding to the subject's chin and also including the absent sidesof the neck and the eye area. The bases, instead of being formed byirregular decagons, are formed by a large square that is constituted by9 detectors.

FIG. 10: shows examples of elongated shapes with a greater or lessernumber of faces, starting from regular pentagons, regular and irregularhexagons. This configuration is better suited to the shape of the headthan the truncated icosahedron or any other shape that mimics thesphere. The shape on the left contains 70 vertices and the one on theright contains 80. The configuration on the left contains 12 regularpentagons, 10 regular hexagons, 15 irregular hexagons, and 105 edges.The configuration on the right contains 12 regular pentagons, 10 regularhexagons and 20 irregular hexagons and 120 edges, and has symmetry ofthe D5d group.

FIG. 11. It shows examples of hollow three-dimensional structures withelongated and narrower shapes in the anterior part corresponding to thearea of the forehead, when the device is in use, than in the posteriorpart corresponding to the nape area when the device is in use, beingtherefore capable of being adapted to the shape of the human head. Thesestructures are made starting from regular pentagons, regular andirregular hexagons. These configurations adapt perfectly to the shape ofthe head, narrower in the front than in the back. The shape on the leftcontains 76 vertices, having the symmetry group Ta, and the one on theright contains 84 vertices.

FIG. 12: shows an example of PET image device of the previous figureperfectly adapted to the shape of the head.

FIG. 13: shows an example of a hexagonal arrangement in “zig-zag” or“armchair” forming a cylinder of arbitrary length to which it isattached, in order to close the cylinder on both sides a hemisphereformed by regular hexagons and pentagons, such as in a truncatedicosahedron. Alternatively, the cylinder is closed by a dome as in FIG.7.

FIG. 14: Left. It shows an example of the disposition of thephoto-detectors of two sizes (in this case they are squarephoto-detectors in which one has sides of half the size of the other) tocover a larger area of the continuous scintillating crystal block withtriangular section, without exceeding this one, since it would collidewith the photo-detectors of adjacent crystal blocks. It has to be notedthat all the large photo-detectors are aligned with each other forming amatrix to facilitate reading by means of rows and columns, as shown inthe figure on the right. Analogous arrangements are made in the casethat the surface of the scintillation crystal block were in the form ofa pentagon, hexagon, etc.

Right. Reading through rows and columns of the photo-detectors of thefigure on the left. The signals from the photo-detectors of each row orcolumn are summed by analog electronics before digitalization. Thesignals of all the smaller size detectors corresponding to each row orcolumn are also summed together with the corresponding signals of thephoto-detectors of large size of the same row or column beforedigitalization. Analogous arrangements are made in the case that thesurface of the continuous scintillation crystal block has the shape of apentagon, hexagon, etc.

FIG. 15: shows an example of arrangement of the photo-detectors in orderto cover a larger area of the scintillating continuous crystal blockwithout the restriction of the alignment in rows and columns. Thisinvolves the individual reading of each of the photo-detectors and theirdigitization by means, for example, of an ASIC (Application SpecificIntegrated Circuit). This arrangement without alignment restrictionsslightly increases the area covered by the photo-detectors. Analogousarrangements are made in the case that the surface of the scintillationcrystal block has the shape of a pentagon, hexagon, etc.

FIG. 16: shows an example of arrangement of the photo-detectors tocompletely cover the surface of the continuous scintillation crystalblock, surpassing the surface area of the triangle. The collision withthe photo-detectors of adjacent crystal blocks, if the latter are squareor rectangular in section, is avoided by increasing the thickness of thetriangular scintillation crystal block, or by slightly shifting (a fewmillimetres) said crystal block towards the outside or by using a thicksheet of material transparent to light, that acts as a diffuser of lightor as a light guide, as shown in the following figures. Analogousarrangements are made in the case that the surface of the continuousscintillation crystal block has the shape of a pentagon, hexagon, etc.

FIG. 17: shows an example of arrangement of the photo-detectors in orderto completely cover the surface of the continuous scintillation crystalblock, surpassing the surface area of the triangle, without therestriction of the alignment in rows and columns. This involves theindividual reading of each of the photo-detectors and their digitizationby means, for example, of an ASIC (Application Specific IntegratedCircuit). This arrangement without alignment restriction minimizes thenumber of photo-detectors that are required to completely cover thecrystal area. Analogous arrangements are made in the case that thesurface of the scintillation crystal block has the shape of a pentagon,hexagon, etc.

FIG. 18: profile view of two adjacent continuous scintillation crystalblocks, the one on the left with a square or rectangular section and theone on the right with a triangular, pentagonal, hexagonal, etc. section.In a dashed line, the photo-detectors are shown. The photo-detectors (9)associated with the crystal block on the right (11) exceed the surfacearea of the triangle in order to completely cover the surface of thescintillation crystal block. The collision with the photo-detectors (8)of the glass on the left (10) is avoided because the glass on the right(11) is slightly (a few millimetres) thicker.

FIG. 19: profile view of two adjacent continuous scintillation crystalblocks, the one on the left with a square or rectangular section and theone on the right with a triangular, pentagonal, hexagonal, etc. section.In a dashed line, the photo-detectors are shown. The photo-detectors (9)associated with the crystal block on the right (11) exceed the surfacearea of the triangle in order to completely cover the surface of thescintillation crystal block. The collision with the photo-detectors (8)of the crystal block on the left (10) is avoided because the glass onthe right (11) is slightly shifted (a few millimetres) outwards.

FIG. 20: profile view of two adjacent continuous scintillation crystalblocks, the one on the left with a square or rectangular section and theone on the right with a triangular, pentagonal, hexagonal, etc. section.In a dashed line, the photo-detectors are shown. The photo-detectors (9)associated with the crystal block on the right (11) exceed the surfacearea of the triangle in order to completely cover the surface of thescintillation crystal block. The collision with the photo-detectors (8)of the crystal block on the left (10) is avoided because a thin sheet (afew millimetres thicker than the glass sheet on the left) of transparentand light diffusing material—or light guide (7)—has been installedbetween the crystal block on the right (11) and its associatedphoto-detectors.

FIG. 21: shows light guides in the form of truncated pyramids (fishtail) with a broader base in the part of the scintillation crystal block(as shown for example in FIG. 21) and with the shape of the polygon ofsaid crystal block, to avoid the collision with the photo-detectors ofadjacent crystal blocks, regardless of the shape of the polygon.

FIG. 22: Example of preferred embodiment in which the detection modulestogether form a structure composed of three parts: the central one is anelongated octagonal base prism whose sides facing the ears are formed bymore modules than the other sides of the octagon, the upper one is adome with a rectangular base that replaces the upper base of the prism,covering it, which exactly closes the previous prism and is thereforeoctagonal on the sides of the ears, and which is placed in the areacorresponding to the upper part of the head when the PET imaging deviceis in use. And the third part is a lower part in the form of a ring orbridge perpendicular to the prism, which replaces the lower base of theprism, and which is a set of several modules arranged in a chain, whichjoin two faces of the prism facing each other, and parallel to eachother, and in a way that this lower part faces the chin of a subjectwhen the device is in use. Each side of the octagonal prism or the domemay be formed by one or more detection modules. Together, the prism anddome form a shape similar to Johnson's solid called elongated squaredome, also called diminished rhombi-cuboctahedron, except that the sidesof the ears have been elongated in the nape-front direction and,therefore, the dome is not square but rectangular. Altogether, the prismand the dome have 17 faces: 5 rectangles, 8 squares (which could also bereplaced by rectangles) and 4 triangles. The lower part, that is placedfacing the chin, can be displaced towards the front half of the prism(so that it covers the lateral area of the head coinciding with theeyes) as seen in FIG. 22, or it can be centered with respect to theprism and the dome that covers the octagonal prism, so that it coversthe central lateral part of the head. In FIG. 22, central drawing, thisarrangement—which is not shown in the figures—would show the detectionmodule of the lower part in contact with the half of each side of themodules immediately above, which constitute faces of the prism.

This configuration, with the lower part of the structure centered hasthe advantage that it allows to observe the central area of the brain,and in particular the hippocampal zone, entorhinal cortex and amygdala.

FIG. 23: Given that the neck has a surface with negative curvature suchas a hyperboloid (a saddle on horseback) it is not easy to approach saidsurface by means of polygons. This figure shows the example of apreferred embodiment shown in the previous figure, but wherein, in thepart of the imaging device disposed facing the chin, crystal blocks inthe form of hexagons and hexagons are used.

FIG. 24: example of a mechanical structure based on carbon fiber, onwhich the different modules with hexagonal, pentagonal, etc. sections,are placed. The thickness of the structure between crystal blocks isexaggerated in the figure so that it can be visualized, but obviously inthe actual implementation it is minimal, to minimize the distancebetween scintillation crystal blocks and maximizing sensitivity.

FIG. 25: This figure shows the crystal block when made out of multipleslabs or semi-monolithic crystals, that is with one dimensionsignificantly larger than the other, coupled to a photosensor andreadout electronics.

-   (12) crystal block-   (13) photosensor array and readout electronics.

DETAILED DESCRIPTION OF THE INVENTION

The object of the present invention is a PET imaging device dedicated tothe examination of the brain, which may be the one of a mammal such as aprimate, and preferably it will be a human brain, for the diagnosis andmonitoring of neurological diseases, maximizing sensitivity and at thesame time minimizing the number of detectors used and, therefore, thecost, weight and complexity.

The sensitivity is optimized by means of a greater angular coverage ofthe detectors, which allows detecting the coincidence of gamma raysemitted in the area of the brain in opposite directions, such as itoccurs in the emission events by PET radioisotopes.

The Instituto Valenciano de Biomecánica (Valencian Institute ofBiomechanics) (IBV) conducted a statistical study of the shape and sizeof the human head. The result is that the size varies significantlyaccording to different nationalities and sexes, and individually withineach country. However, a common feature is that the shape of the humanhead is not spherical but is narrower in the direction between the twoears, compared to the other two directions perpendicular to it (seeFIGS. 1 and 2). This suggests the design of a PET imaging device with anelongated geometry to better adapt it to the shape of the human head.The object of the present invention is a PET device that meets theseprovisions, that is, maximum angular coverage to maximize sensitivity,and elongated shape, similar to the head, in particular the human head,placing the PET imaging device as close as possible to the brain, tominimize the number of detectors used. Another common feature is thatthe head is narrower in the front than in the back (FIG. 2 right).

An essential feature of the PET imaging devices of the invention istherefore that the gamma detection modules are arranged so that they cansurround the head of the subject, together forming an elongated,non-spherical surface, to better adapt to the three-dimensional shape ofthe head, in particular the human head. This allows to obtain a maximumsensitivity while minimizing the number of detectors used and,therefore, the cost of the PET imaging device.

However, and given that the PET imaging device is designed for thediagnosis of neurological diseases, many patients will be elderly, andsome of them, or younger ones may present mental illnesses, such asschizophrenia, it is critical for a correct examination that the patientis comfortable and that he/she can see, hear and breathe without anydifficulty. On the other hand, the sizes and shapes of the heads differon average between different continents and nationalities, andindividually within each country, with total variability in the size ofup to around 5%. Therefore, the proposed designs should allow a spacebetween the image device and the head sufficiently loose to fitdifferent sizes and shapes of heads.

The device of the present invention is a PET imaging device dedicated tothe observation of the brain, characterized in that as a whole it has astructure with a shape capable of housing a head, comprising independentgamma ray detection modules, said detection modules comprise continuousscintillation crystal blocks of polygonal main section, in which thedetection modules together form a hollow three-dimensional structurecapable of circling the head, and said elongated three-dimensionalstructure having a major axis in the direction corresponding to thefront-nape direction and a shorter axis in the direction correspondingto the straight line joining the ears, and the adjacent scintillationcrystal blocks being arranged to fit laterally in an exact manner witheach other throughout their entire thickness, building a mosaic, that isto say, without leaving gaps and without overlapping each other.Preferably, the adjacent scintillation crystal blocks are of the samethickness fitting laterally in an exact manner with one anotherthroughout their entire thickness.

According to particular embodiments, in the PET imaging device, thedetection modules have a square or rectangular shape, forming together alying down, hollow, prism with a rectangular base (as shown for examplein FIG. 5) capable of housing a head, the anterior base of said prismbeing in the front part of the structure that can be confronted to theface of a subject, and the rear base of the prism is in thearea—corresponding to the back of the head-, is in the posterior part ofthe structure that can be confronted to the occipital bone; andoptionally, the rear base can be extended with additional detectionmodules towards the area corresponding to the back when the device isplaced on the head of a subject.

According to further particular embodiments, in the PET imaging devicethe detection modules have a square or rectangular shape, formingtogether a lying down, hollow, prism with a rectangular base capable ofhousing a head, the anterior base of said prism being at the front ofthe structure that can be confronted to the face of a subject, and therear base of the prism is in the area that corresponds to the back ofthe head, at the back of the structure that can be confronted to theoccipital bone, each side of said prism being covered with detectorswith continuous crystal blocks of square section, so that each side ofthe prism that can be placed facing the neck, and the front base, arecovered only partially, so that the neck fits loosely and does notobstruct the vision, while the rear base and all remaining sides of theprism are completely covered by detectors; and optionally, the rear basecan be extended with additional detection modules towards the areacorresponding to the back when the device is placed on the head of asubject.

For any of these described embodiments of the PET image device, in whichthe detection modules have a square or rectangular shape formingtogether a lying down, hollow, prism with a rectangular base, capable ofhousing a head, the anterior base of said prism being in the front ofthe structure that can be confronted to the face of a subject, and theposterior base of the prism being in the area that corresponds to theback of the head, at the back of the structure that can be confronted tothe occipital bone, wherein the side of the prism that can be confrontedto the neck when the imaging device is in use, and which is onlypartially covered with detection modules, forms with the front base ofthe prism a fragment thereof with an “L” form, and this L-shapedfragment corresponding to the chin area when the device is in use, isadjustable in position for each subject after having placed the PETimaging device on the head. In the position adjustment operation for aparticular subject, the entire “L” shaped fragment can be displaced.

According to further particular embodiments, the detection modules havea square or rectangular shape, forming together a hollow prism of basesof pentagonal, hexagonal, octagonal etc. section, capable of housing ahead, the anterior base of said prism being at the front of thestructure that can be placed in front of the face of a subject, and theother base of the prism on the back of the structure that would face theback of the head, that could be confronted to the occipital bone. Anexample of this embodiment is shown in FIG. 6, wherein the squarecontinuous scintillating crystal blocks are arranged in the form of anirregular eight-sided prism. The sides corresponding to the lower areaof the jaw only have detectors in that region to leave a loose space forthe neck. The rear base has been completely covered with squaredetectors. The front base has been covered only partially, in the areaof the chin and forehead, to allow the eyes a comfortable vision.

According to further particular embodiments, the PET imaging device maycomprise triangular, square and rectangular shaped detection modules,which together form a hollow prism with a base in the form of apolygonal dome, for example square, pentagonal, hexagonal or octagonaldome.

According to additional particular embodiments, the detection moduleshave a square or rectangular shape forming together a hollow prismcapable of housing a head, wherein polyhedral domes form the bases. Inparticular, for this alternative of the invention, the detection moduleshave a square or rectangular shape forming together an octahedral prismcapable of housing a head, and with a base in the form of a square dome.

In another embodiment of the present invention, the bases of the prismshave a polygonal dome shape. An example of a square dome is shown inFIG. 7. This base adapts better to the shape of the back of the head(FIG. 8), allowing to significantly decrease the number of continuousscintillation crystal blocks used. However, it has the disadvantage ofadding continuous scintillation crystal blocks of triangular surface.Likewise, the front part has also been covered, although only partially,in the area of the chin and the forehead, to allow the eyes acomfortable vision, by means of a base in the form of a square dome.Other similar bases in the form of a pentagonal, hexagonal dome, etc.,can also be implemented analogously by means of introducing crystalblocks of irregular polygonal section.

According to additional particular embodiments of the PET imagingdevice, the detection modules form a hollow prism the lateral faces ofwhich are the faces that can be arranged between the nape and theforehead of a subject, and the bases of the prism are the faces that canbe arranged parallel to the ears, as shown in FIG. 9. In the example theprism is formed by eight sides (octagonal prism), including the sidecorresponding to the chin and the absent sides of the neck and the eyearea. The bases, instead of being formed by irregular octagons, areformed by a large square that is constituted by 9 detectors.

Forms that approach the sphere starting from flat surfaces can beachieved by hexagons and pentagons as in the truncated icosahedron.However, the size of the truncated icosahedron is limited by the size ofthe edge of the faces, and is given by the following formula, for anedge of unit size:

$r_{5} = {\frac{1}{2}\sqrt{\frac{1}{10}\left( {125 + {41\sqrt{5}}} \right)}}$$r_{6} = {\frac{1}{2}\sqrt{\frac{3}{2}\left( {7 + {3\sqrt{5}}} \right)}}$where r₅ y r₆ are the radius to the center of the pentagons andhexagons, respectively. For an edge of size a we must multiply r₅ and r₆by the length of the edge.

Bearing in mind that continuous scintillation crystals are manufacturedfrom cylindrical ingots with a maximum diameter of around 70 mm in thecase of LSO, or 135 mm in the case of BGO, the maximum edge of a regularhexagon (corresponding to half of the ingot diameter) will be 163 mm.This limits the truncated icosahedron shape to the BGO case, andconsequently the LSO and its variants cannot be used. The family of LSOcrystals has the characteristic of emitting a large amount of light anda very short emission time with respect to the BGO, which are verysuitable for the precise determination of the time of flight (TOF: Timeof Flight).

Alternatively, in another embodiment of the present invention, the PETimaging device has elongated shape starting from 12 flat surfaces ofpentagonal section and different number of flat surfaces of—regular andirregular—hexagonal section, of arbitrarily large size. In FIG. 10 twoexamples are shown. In the example on the left, there are 70 vertices,12 pentagons, 15 regular hexagons and 10 irregular hexagons, that is, 37faces in total. This form, in which it is critical to add irregularhexagons, has two important advantages with respect to the truncatedicosahedron (the soccer ball): 1) it has an elongated shape that adaptsbetter to the head; 2) it can be made arbitrarily large. Similarly, theconfiguration on the right of the same FIG. 10 contains 12 regularpentagons, 10 regular hexagons and 20 irregular hexagons and 120 edges,and has symmetry of the D5d group.

Alternatively, starting from 12 pentagons and different number ofregular and irregular hexagons it is possible to put into practicepractically any elongated shape.

Alternatively, another embodiment of the present invention relates to aPET imaging device with an elongated shape, which is also narrower inthe forehead area than in the rear part, adapting even more to the shapeof the head. In FIG. 11 two examples are shown. The example on the leftcontains 76 vertices, having the symmetry group Ta, and the one on theright contains 84. Starting from 12 flat surfaces of pentagonal sectionand different number of flat surfaces of hexagonal, regular andirregular, section, these and other shapes of arbitrarily large size canbe put into practice. In FIG. 12, the arrangement of continuous crystalblocks of the PET imaging device, perfectly adapted to the shape of thehead, is shown.

Starting from regular polygons of a few types can also be used toconstruct Archimedean solids other than the truncated icosahedron, someof which could be used as helmets if only half the solid is used, forexample: truncated Icosidodecahedron or the dull Dodecahedron.

Starting from regular polygons of a few types, Johnson's solids can alsobe constructed, some of which could be used as helmets if the base isremoved (the largest polygon, octagon or decagon), for example:elongated square dome, elongated pentagonal dome, elongated pentagonalroundabout, gyro-elongated square dome, gyro-elongated pentagonal dome,gyro-elongated pentagonal roundabout, pentagonal ortho-roundabout-dome,(Ortocupularrotonda), pentagonal gyro-dome-roundabout, diminishedrhomboicosahedron, diminished paragyroid rhomboicosahedron, anddiminished metagiroide rhomboicosahedron.

Also starting from simple irregular polygons one can build the Catalansolids, some of which could be used as helmets if only half of the solidis used, for example: deltoidal hexecontahedron and pentagonalhexecontahedron.

These configurations from polygons such as pentagons, regular hexagonsand irregular hexagons, etc., are not easy to build. To this purpose,according to a particular embodiment, a mechanical system based on arigid honeycomb like structure is provided (FIG. 24) where each one ofthe detection modules will be housed in their pre-established positionand orientation. Each honeycomb cell has the proper geometry and size tocomplete the final configuration of the device. The matrix alsoincorporates a mechanical interface to be able to link this element withother possible components of the system (positioner of the detectorsystem with respect to the patient, support, protective or embellishercasing). The matrix is opaque to visible light, to prevent light fromentering the cell, but the light is low density to avoid unwantedinteractions (scatter, Compton, attenuation) with radiation from thepatient.

The PET imaging device of the present invention, in addition to thegamma ray detection modules, comprises the electronics for theacquisition of data coming from the detection modules, the electronicsof the temporal coincidences “trigger”, a computer for the dataacquisition and storage and computer programs for reconstruction andvisualization of the image from the data.

Each detection module comprises a continuous scintillation crystalblock, also called monolithic crystal block, a set of photo-detectorsfor the collection of the light emitted by the scintillation crystalblock after the interaction of the gamma ray thereon, the readingelectronics of said photo-detectors, and optionally, a mechanicalencapsulation of the whole assembly, as shown, by way of example, inFIG. 3. In this particular embodiment shown in this figure at least oneof the crystal faces is covered at least in part by the set ofphoto-detectors.

On the other hand, it must be taken into account that thephoto-detectors must be placed on flat surfaces of the continuousscintillation crystals, or of light diffusers or light guides attachedto said flat surfaces. The reason is that the photodetectors used in thepresent invention, both solid state photodetectors and positionsensitive photomultipliers, have flat entry surfaces. Therefore,scintillation crystal blocks with curved surfaces are not used in thePET device of the invention since they would not fit perfectly to theflat surface of the photo-detectors. In addition, it is much morecomplicated to build continuous crystal blocks with curved surfaces.Therefore, each of said crystal blocks has a polygonal main section, forexample, in the form of a polygonal prism or truncated polygonal pyramid(see, by way of example, FIG. 4). In general, the width of the crystalblock will be the same for all scintillation detectors regardless oftheir polygonal shape.

In a preferred configuration, each detection module has a single facewhere the photo-detectors are located. This face is covered to themaximum by photo-detectors in order to optimize the resolution in impactposition of the gamma rays in the crystal block, the time resolution andenergy resolution. In said preferred configuration, the surface oppositeto the one of photo-detection is completely polished and covered by aretroreflector. The detection modules can be arranged so that the gammarays enter on the opposite side to the photo-detectors or,alternatively, on the side where the photo-detectors are located.

It should also be borne in mind that virtually all photo-sensors have aphotosensitive surface of square or rectangular section, especiallysolid-state photo-sensors, and it is therefore very convenient that thecontinuous scintillation crystal blocks also have a square orrectangular section where the photo-sensors are placed. In this way, thenon-sensitive (dead) zones are minimized and therefore the lightcollection is maximized, optimizing the energy resolution of the gammaray and its impact position on the crystal block.

Furthermore, to maximize the sensitivity of the detector it is criticalthat there are no gaps between the scintillation crystal blocks so thatthe gamma rays do not escape between said gaps and be therefore notdetected. At the same time, it is important to avoid overlapping betweenthe scintillation crystal blocks, i.e. some crystal blocks are mountedon others, to minimize the cost of the PET imaging device. Therefore, itis very convenient that all the lateral faces of the crystal blocks areexactly coupled to their neighbouring crystal blocks.

Therefore, the PET devices of the invention have in common that thedifferent continuous scintillation crystal blocks do not leave gapsamong them by which the gamma rays can escape without being detected,except the minimum width of the encapsulation in case there is anencapsulation that contains each module. Therefore, an essential featureof the present invention is that the scintillation crystal blocks havethe shape of polygons, preferably of the same thickness, fittinglaterally in an exact manner with one another throughout their entirethickness, building a mosaic, i.e, without leaving gaps and withoutoverlapping each other.

It should also be borne in mind that it is not appropriate for themanufacture of a PET imaging device that the shape of the modules bedifferent for each detection module. Optimal is that all the detectionmodules are identical to each other to facilitate the manufacture ofthem in series. It would also be assumable in manufacturing, that thereonly were be two or three types of different detection modules. This isimportant not only for the manufacture of the detection modules but alsofor the method of determining the impact point of the gamma ray in thecrystal block, which is different for each type of module. Therefore, ifan embodiment consists of crystal blocks in many different forms, itwould be necessary to develop an algorithm for determining impact pointof the different gamma ray for each crystal block.

Each detector module can be enclosed by an encapsulation to immobilizethe module components (continuous crystal block, photo-detector andassociated electronics) in their nominal position, avoiding that lightbe introduced inside the detector module and at the same timedissipating the heat generated by the detector module components.

The materials to be used for said components can be of different nature(such as polymers or metals), and their selection will depend on factorsinherent to the selected configuration and the density of the detectionmodule elements, but preferably polymeric materials are used due totheir scarce interaction with the patient emitted radiation.

The manufacture of the components is carried out by any method suitableto the chosen materials (machining, injection, casting, printing,sintering) but is preferably carried out by 3D printing or sintering dueto the low production cost for small series, the variety of availablematerials and finishings, as well as the limited restrictions of thesemethods in terms of geometric design characteristics and neededtolerances.

All commercial and academic PET use detection modules (and scintillationcrystals) of square or rectangular section. Furthermore, in said modulesthe light produced by the impact of the gamma rays on the crystal isdirected towards a single photo-detection surface, of square orrectangular section. Said surface of square or rectangular section canbe completely covered by photo-detectors of square section to maximizethe resolution in position, in energy and time.

However, it is not possible to completely cover flat surfaces oftriangular, pentagonal, hexagonal, or heptagonal section, of thecontinuous scintillation crystal blocks that are used in the presentinvention, by commercially available square section photo-detectors.This involves a deterioration in the resolution in position, energy andtime. To overcome this difficulty that has not been raised up to now,the present invention provides different solutions that are developedbelow.

An object of the present invention is the use of two sizes ofphoto-detectors, for example, squares of 6 and 3 mm, as shown in FIG. 14to maximize the area covered by the detectors without exceeding it,since it would collide with the photo-detectors of adjacent continuouscrystal blocks. It has to be noted that all large-sized photo-detectorsare aligned with each other forming a matrix to facilitate reading byrows and columns, as shown in the next figure. Analogous arrangementsare made in the case that the surface of the scintillation crystal blockis in the form of a pentagon, hexagon, etc.

Alternatively, an electronic method is available to read and digitizeeach and every one of the photo-detectors Individually by, for example,an ASIC (Application Specific Integrated Circuit), it is possible tocover a larger area of the scintillating crystal block when therestriction of alignment in rows and columns disappears. Analogousarrangements are made in the case that the surface of the scintillationcrystal block were in the form of a pentagon, hexagon, etc.

Alternatively, the squares of the photo-detectors may protrude if theadjacent surface is a square or rectangular surface. This can be done byusing slightly thicker crystal blocks (for example, as shown in FIG.16).

The photo-detectors can be arranged to completely cover the surface ofthe continuous scintillation crystal block surpassing the surface areaof the triangle. The collision with the photo-detectors of adjacentcrystal blocks, if the latter are square or rectangular in section, canbe avoided by increasing the thickness of the triangular scintillationcrystal block (FIG. 18) the glass on the right is slightly shifted (afew millimetres) towards the outside (FIG. 19) or by using a thick sheetof material transparent to light, which acts as a light diffuser or as alight guide (FIG. 20). Analogous arrangements are made in the case thatthe surface of the scintillation crystal block were in the form of apentagon, hexagon, etc. In general and for any embodiment, the collisionwith the photo-detectors of adjacent crystal blocks can also be avoidedregardless of the shape of the polygon by using light guides in the formof truncated pyramids (fish tail) with a wider base in the part of thescintillation crystal block (as shown for example in FIG. 21) and withthe polygonal form of said crystal block. Said narrowed light guides canbe obtained starting from optical fibers.

The conditions fulfilled by the PET devices of the invention are thefollowing: 1) they are elongated shape, adapted as much as possible tothe shape of the head, constituted from independent detection modules,with the greatest possible angular coverage of the brain; 2) maximumproximity of the detection modules to the head, respecting the comfortof the patients; 3) gamma ray detection modules composed of continuousscintillation crystal blocks with flat surfaces; 4) maximum threedifferent forms of scintillation crystal blocks; 5) exact couplingbetween the lateral surfaces of the scintillation crystal blocks; and 6)surfaces that can be covered almost completely with square surfacephoto-sensors.

The PET imaging device according to any of the alternatives describedabove, may comprise a protective element capable of being mechanicallyor manually operated to produce a complete adaptation of the PET imagingdevice to the shape of an object, such as a head, the image of which isto be obtained, and to immobilize said object.

Said protective element can be selected between an air cushion system,attached to the set of detection modules, and a system of elasticcontainers, filled with small spherical particles or with any othergeometry, of low density.

The movement of the head during the acquisition time generates artefactsand degrades the image quality. Two methods are proposed in the presentinvention to reduce such an effect by introducing an element between thethree-dimensional structure of the detector array and the patient'shead, with the ability to dynamically adapt to both.

The present invention also relates to a method to carry out an imageacquisition with a PET imaging device defined above.

The present invention also relates, according to particular embodiments,to a method for carrying out an image acquisition with a PET imagingdevice defined above, comprising arranging a protective element betweenthe PET imaging device and the object, such as a head, the image ofwhich is intended to be obtained, such that said protective element whenbeing mechanically or manually operated produces a complete adaptationof the PET image device to the shape of the object of which is intendedto be obtained. The protective element can be an air cushion system or asystem of elastic containers, as defined above.

In the case of the air cushion system, attached to the three-dimensionalstructure of the detector assembly and located between saidthree-dimensional structure and the patient's head, which can beoperated by means of a manual or automated inflation system, whichallows filling the separation between the three-dimensional structureand the head, immobilizing the patient's head inside the structure.

In the case of the bag system, for example two, or elastic containers,filled with small spherical particles or with any other geometry, of lowdensity—to avoid artefacts—, they can be slightly mechanically pressedto the front and side of the patient's head to, then they are pressedunder vacuum, using a pump included for that purpose, which causes thesebags or containers to faithfully adopt the shape of the patient's skull,preventing it from moving or rotate with regard to the original positionin which it was at the time the vacuum was applied.

EXAMPLES OF PREFERRED EMBODIMENT Example 1

In a first preferred embodiment of the brain PET imaging device, thecontinuous scintillation crystal blocks together form an 8-sided prismof elongated shape, and which can be arranged inclined along the head,as shown in FIG. FIG. 22 This prism is closed at the top by a domecomposed of 8 square-section crystal blocks and 4 triangular-sectioncrystal blocks. At the bottom, the prism is closed by a bridge formed by5 square section scintillation crystal blocks around the chin.

The scintillation crystal blocks are BGO (Bismuth Germanate, Bi₄Ge₃O₁₂)and with truncated pyramidal form with a square section, 20 mm thick and95×95 mm² base size. The truncation of the pyramid is determined by theshape of the prism in such a way that the sides of the crystal blocksfit perfectly with each other to prevent gamma rays from escaping.

The base of each crystal block is completely covered with SiPM-typephoto-detectors of 6×6 mm³ size. All the electronic signals produced inthe photo-detectors are read by rows and columns configuration. In thecase of the scintillation crystal blocks with triangular section, thephotodetectors are placed as in FIG. 16 and a light diffusing sheet or alight guide is placed between the crystal block and the photo-detectorsas shown in FIG. 20 On the opposite side to the photo-detectors, aretro-reflector is placed to maximize the detection of light whilepreserving its original distribution.

Example 2

In a second preferred embodiment, the continuous scintillation crystalblocks are LSO (Lutetium Oxi-Orthosilicate doped with Cerium). All thecrystal blocks have a 20 mm width. 5 crystal blocks have a regularhexagonal section of 35 mm edge. 6 crystal blocks have a pentagonalsection of the same edge. 10 crystal blocks have an irregular hexagonalsection with 4 edges of 35 mm and two slightly longer edges. This shapeand size of the crystal blocks has the advantage of maximizing the sizeof the LSO ingot. All these crystal blocks are joined together to forman elongated structure around the patient's head as shown in FIG. 12.

The photo-detectors are of 3×3 mm² size and the signals produced by themare read individually through an ASIC that digitizes both intensity andtime. This allows obtaining time of flight information, obtaining abetter image quality.

The invention claimed is:
 1. A PET imaging device for the observation ofa brain, which has, as a whole, a hollow three-dimensional structurewith a shape capable of housing a head, the PET imaging devicecomprising a plurality of independent gamma ray detection modules thattogether form a structure capable of surrounding the head, saiddetection modules comprise continuous scintillation crystal blocks,wherein “continuous” means that the crystal blocks can be continuous inone or in two directions, wherein each of the continuous scintillationcrystal blocks has a polygonal main cross-section, and wherein saidstructure is an elongated structure having a major axis in a directioncorresponding to the front-nape direction and a shorter axis in adirection corresponding to a straight line joining ears on the head, andthe continuous scintillation crystal blocks are positioned adjacent tofit laterally in an exact manner with each other throughout their entirethickness, building a mosaic, without leaving gaps between adjacentcrystal blocks and without overlapping each other.
 2. The PET imagingdevice according to claim 1, wherein the main cross-section of each ofthe scintillation crystal blocks has a square or rectangular shape, andwherein the structure formed from the scintillation crystal blocks is ahollow, rectangular base prism, an anterior surface of said prism, beingin a front part of the structure, that can be confronted to a face of asubject, and a rear surface of the prism, being in a posterior part ofthe structure that can be confronted to an occipital bone of thesubject.
 3. The PET imaging device according to claim 2, wherein eachside of said prism is covered with detectors with a square main crosssection; an insertion side of said prism, the insertion side having anopening through which the head may be inserted, being covered onlypartially, such that the neck fits loosely; the anterior surface being,covered only partially, such that the vision of the subject is notobstructed, and the back base and all remaining sides of the prism arecompletely covered by the detection modules.
 4. The PET imaging deviceaccording to claim 2, in which the rear base has been extended withadditional detection modules towards an area corresponding to a back ofthe subject when the device is placed on the head of the subject.
 5. ThePET imaging device according to claim 2, wherein an insertion side ofthe prism is only partially covered with detection modules, wherein theinsertion side forms with a lower portion of the anterior surface of theprism a fragment thereof in the form of “L”, and this L shaped fragment,that corresponds to a chin area of the subject when the device is inuse, is adjustable in position for each subject after the PET imagingdevice has been placed on the head.
 6. The PET imaging device accordingto claim 1, wherein the main cross-section of each of the scintillationcrystal blocks is square or rectangular shaped, and wherein thestructure formed by the detection modules is a hollow prism with a baseof pentagonal, hexagonal, or octagonal section, an anterior base of saidprism being on a front part of the structure capable of being confrontedto a face of a subject, and the posterior base of the prism is at a backof the structure that would be facing a rear part of an occipital boneof the subject.
 7. The PET imaging device according to claim 1,comprising detection modules having scintillation crystal blocks whereinthe main cross-section is triangular, square, rectangular shape, orcombinations thereof, and forming together a hollow prism with apolygonal dome shaped base.
 8. The PET imaging device according to claim7, wherein a shape of the dome is selected from square, rectangular,pentagonal, hexagonal or octagonal.
 9. The PET imaging device accordingto claim 8, wherein the structure includes three parts: a central partwhich is configured as an elongated octagonal based prism, the sides ofthe central part are configured to face ears of a subject and are formedby more modules than the other surfaces of the octagonal based prism, anupper part is a rectangular base dome that defines an upper base of theoctagonal based prism, covering it and closing the prism wherein theupper part is arranged in the area corresponding to an upper part of ahead of the subject when the PET imaging device is in use, and a thirdpart is a lower part of the structure in a ring or bridge connected tothe central part perpendicular to the prism, and the third part defineda lower base of the prism, and a set of several detection modulesarranged in a chain which join two faces of the prism confronted andparallel to each other, and the third part is configured such that itfaces a chin of a subject when the device is in use.
 10. The PET imagingdevice according to claim 9, wherein the lower part which is placedfacing the chin is shifted towards an anterior half of the prism or iscentered with respect to the prism and the dome covering it, such thelower part is capable of covering a central lateral portion of the head.11. The PET imaging device according to claim 9, wherein the detectionmodules form an octagonal base hollow prism which in the part that is tobe placed on the upper part of the head has the form of a square orrectangular dome, and the lower part that is to be disposed opposite thechin is covered with crystal blocks with the form of heptagons andhexagons.
 12. The PET imaging device according to claim 8, wherein thedetection modules have a square or rectangular shape, forming together alying down, hollow, octahedral prism and with a square or rectangulardome shaped base.
 13. The PET imaging device according to claim 1,wherein the detection modules form a prism, the lateral faces of whichare capable of being arranged between a nape and a forehead of asubject, and the bases of the prism are the faces capable of beingarranged parallel to the ears.
 14. The PET imaging device according toclaim 13, wherein the prism is formed by eight sides, the eight sidesincluding: two sets of 3 adjacent sides, each set configured to face oneof the ears on the head, and forming an interior angle between adjacentsides corresponding to an interior angle of an octagon, these two setsare separated by a first side, forming interior angles with the adjacentsides which correspond to the interior angle of an octagon, and capableof facing an upper part of the head, a second side, parallel to thefirst side, forming interior angles with the adjacent sides whichcorrespond to the interior angle of an octagon capable of facing thelower part of the chin, and comprising a gap for placing the PET overthe head, an anterior base facing a neck of the subject; and a posteriorbase facing an ocular area of the subject, and the bases are squareshaped and are made up of several detectors.
 15. The PET imaging deviceaccording to claim 1, wherein the detector modules form a hollowthree-dimensional structure having an elongated shape comprising 70vertices or 80 vertices, the structure having D_(5d) symmetry, and thedetector modules having flat surfaces wherein at least a portion thedetector modules have a pentagonal or hexagonal cross-section, regularor irregular.
 16. The PET imaging device according to claim 1, whereinthe detector modules form a hollow three-dimensional structure having anelongated shape that is further narrower in an anterior part,corresponding to a forehead of a subject when the device is in use, thanin a rear part corresponding to a nape area of the subject, when thedevice is in use, such that it is capable of being adapted to the shapeof a head.
 17. The PET imaging device according to claim 16, wherein thedetector modules form a hollow three dimensional structure comprising 76vertices or 84 vertices, the structure having Ta symmetry, and thedetector modules having flat surfaces, wherein at least part of saiddetector modules have a pentagonal or hexagonal, regular or irregular,cross-section.
 18. PET imaging device according to claim 1, whichfurther comprises a mechanical matrix structure opaque to visible light,rigid, honeycomb like, wherein each of the detector modules are housedin a pre-set position and orientation; and a mechanical interface forconnecting a module to the other ones.
 19. The PET imaging deviceaccording to claim 1, that comprises continuous scintillation crystalblocks of the same width for all of the detectors, regardless of theirpolygonal shape.
 20. The PET imaging device according to claim 1,comprising detection modules of two different sizes.
 21. The PET imagingdevice according to claim 1, wherein each detection module has a uniqueface where the photosensors are located and such that a surface oppositeto photo detection is completely polished and covered by a retroreflector.
 22. The PET imaging device according to claim 1, comprising alight diffusing sheet or a light guide between each continuousscintillation crystal block and photo detectors.
 23. The PET imagingdevice according to claim 1, comprising a light guide, between eachcontinuous scintillation crystal block and the photo detectors, in theform of truncated based pyramids with a base wider at the portion of thescintillation crystal block, and with the shape of the crystal blockpolygon, to prevent the photo detectors from adjacent crystal blocksfrom colliding with each other.
 24. The PET imaging device according toclaim 1, wherein the detection modules are arranged so that gamma raysenter the face opposite the photo detectors.
 25. The PET imaging deviceaccording to claim 1, which comprises a protective element capable ofbeing mechanically or manually actuated and capable of producing acomplete adaptation of the imaging device to a shape of an object, theimage of which is to be obtained, and to immobilize said object.
 26. ThePET imaging device according to claim 25, wherein the protective elementis selected from an air cushion system, attached to the array ofdetection modules, and a system of elastic containers, filled withparticles.
 27. A method for obtaining images with a PET imaging devicededicated to observation of a brain of a subject, which has a structurewith a shape capable of housing a head of the subject, the methodcomprising the steps of detecting gamma rays within independent gammarays detection modules, said detection modules comprise continuousscintillation crystal blocks of polygonal main cross-section, wherein:the detection modules define a hollow three-dimensional structurecapable of receiving the head, the elongated three-dimensional structurehas a major axis in the direction between a front of the head and a napeof the subject, the elongated three-dimensional structure has a shorteraxis in the direction corresponding to a straight line between ears ofthe subject, and the adjacent scintillation crystal blocks arepositioned adjacent to each other to fit laterally in an exact mannerwith each other throughout their entire thickness, building a mosaic,without leaving gaps between adjacent crystal blocks and withoutoverlapping each other.
 28. The method according to claim 27, comprisingproviding a protective element between the PET imaging device and anobject the image of which is to be obtained, such that said protectiveelement upon being mechanically or manually actuated produces a completeadaptation of the PET image device to a shape of the object.
 29. Themethod according to claim 28, wherein the protective element is an aircushion system, attached to the array of detection modules, which byusing an inflation system fills the gap between the PET image device andthe object the image of which is to be obtained.
 30. The methodaccording to claim 28, wherein the protective element is a system ofelastic containers, filled with particles that upon being mechanicallypressed cause adaptation of the PET imaging device to the shape of theobject the image of which is to be obtained.